The art has long recognized that adsorbents are capable of distinguishing between the diverse molecules present in air and therefore, that with appropriate molecular sieves, and adsorption and desorption techniques, air can be fractionated. Reference is made to U.S. Pat. Nos. 4,011,065 and 4,015,956 for detailed discussion to the separation of air into an oxygen enriched stream and a nitrogen enriched stream through the use of carbon molecular sieve (solid phase) adsorbents arranged in a multitower reversing system of the pressure swing type, with the individual towers being operated cyclically through an adsorption cycle and a desorption cycle.
By and large, pressure swing adsorption systems (PSA) employed to fractionate air into a high purity nitrogen stream and an oxygen enriched (dump) stream, have heretofore largely ignored the presence of the several minor constituents in air, including notably argon and water vapor. The argon largely ends up in the nitrogen stream. The water vapor ends up in the oxygen stream.
In particular, the water vapor content of air has been ignored for reason that the nitrogen product stream is dry. The water vapor is adsorbed by the carbon molecular sieve adsorbent during the adsorption cycle along with the oxygen and then is desorbed along with the oxygen during the purge or regeneration of the adsorbent in the course of the desorption cycle. Indeed, presence of water vapor in the feed air has been ignored to such an extent by the art that discussions of the PSA nitrogen systems in the literature alternatively suggest a self-contained PSA add-on to existing plants (without any water vapor control), or solely because such is cheaper, the use of plant compressed air (which sometimes is dried after compression) if the PSA system will be part of a new plant. However, ignoring the water vapor content of the feed air has significantly reduced efficiency of the PSA high purity nitrogen process because presence of water vapor in the air stream is in fact much more disadvantageous than the art has recognized heretofore.
Thus, it has been discovered by the inventors hereof, that drying of the air prior to entry into the PSA towers is desirable. Drying the compressed air by a solid desiccant pressure swing air dryer integrated operationally with the pressure swing adsorber of a PSA high purity nitrogen separation system is particularly advantageous.
In addition, it has been discovered by the inventors hereof that passage of high purity nitrogen back into the PSA tower being regenerated for purge purposes is most advantageous. Insofar as the inventors hereof are aware the PSA art has not appreciated the desirability of using a purge gas.